Production of plasma from a microwave electromagnetic field can be carried out using different mechanisms, depending on the pressure range considered, and whether or not a static magnetic field (as opposed to the magnetic component of the microwave electromagnetic field) is present. Below one torr (1 torr=133 Pa), in the presence of a sufficiently intense magnetic field, production of plasma by microwaves can be obtained in resonant coupling mode, called electron cyclotron resonance (ECR). At electron cyclotron resonance, the electrons are very effectively accelerated by the microwave electric field if the intensity of the magnetic field (which can be produced by coils or permanent magnets) is such that the frequency of gyration of the electrons in the magnetic field is equal to the frequency f0 of the microwave electric field, hence:f0=eB0/2πme  (1)where me is the mass of the electron, −e is the charge of the electron and B0 the intensity of the magnetic field corresponding to electron cyclotron resonance (ECR) for the microwave frequency f0.
In the absence of collisions, the trajectory of the so-called fast electrons thus accelerated at electron cyclotron resonance in the magnetic field, then coils in a helical motion around a magnetic field line, and each electron can thus oscillate between two mirror points where the speed of the electron parallel to the magnetic field line cancels and changes sign. Indeed, when the intensity of the magnetic field increases due to conservation of the magnetic moment of the electron on its trajectory (adiabatic invariant of the motion), the speed of the electron parallel to the field line decreases in favor of the speed of rotation about the magnetic field line (conservation of kinetic energy of the electron) until it cancels at the first mirror point, then changes direction heading for the second mirror point M, and so on. These mirror points M, where the intensity of the magnetic field is identical, are generally located facing two opposite magnetic poles of the same magnet (as in FIG. 1) or of two adjoining magnets (see FIG. 2).
FIG. 1 illustrates a permanent magnet m1 with its magnetization direction shown schematically by the arrow. Two field lines L are shown and the trajectory T of an electron, which is driven in rotation about a field line L, is shown schematically on one of them. FIG. 2 illustrates two permanent magnets m1 and m2 arranged parallel to one another and with opposite magnetization directions, shown schematically by the arrows. In this case, the field lines L connect the adjoining poles of the two magnets, the trajectory of an electron being shown schematically on one of them.
The motion described above continues until the fast electron accelerated at electron cyclotron resonance undergoes collisions, and particularly ionizing collisions. The plasma, that is the electrons and the ions thus produced along the trajectories of the fast electrons accelerated at electron cyclotron resonance, then diffuses, by successive collisions, to either side of these trajectories, hence perpendicularly to the field lines.
Several devices using electron cyclotron resonance are described in documents FR 2 797 372, FR 2 838 020, FR 2 904 177, FR 2 904 178 and FR 2 938 150. Generally, these devices include magnetic structures with permanent magnets positioned at the exit of a coaxial microwave applicator (microwave applicator). These magnetic structures are such that all the fast electrons accelerated at electron cyclotron resonance oscillate between two mirror points M without encountering material surfaces capable of collecting them.
Thus the microwave power absorbed for accelerating them is entirely dedicated to plasma production by ionizing collisions, and not to bombarding surfaces interrupting their trajectory, giving an optimum efficiency with respect to plasma production by the fast electrons. All these devices have allowed microwave plasmas to be produced with excellent performance. At a higher pressure, above one-tenth of a torr (13.3 Pa) or one torr (133 Pa), that is when the frequency of collisions v of the electrons in the plasma reaches the order of magnitude of the microwave pulsation ω (v≈Ω), electron cyclotron resonance becomes less effective and heating of the electrons, that is their progressive acceleration in the microwave electric field, is accomplished for example by collision absorption immediately upon leaving the applicator. In this operating mode at higher pressure, above a few Pa, it is then necessary to avoid production inside the coaxial applicator by separating the volume under reduced pressure from the atmospheric pressure by a fluid-tight dielectric window directly at the exit of the applicator. Reference can be made in this regard to document FR 2 840 451.
Despite improvements applied to microwave applicators by the abovementioned documents, all these applicator have two major shortcomings, to wit:                on the one hand, the fast electrons accelerated to electron cyclotron resonance remain trapped near the walls by the magnetic field between two mirror points situated near the poles: plasma production thus remains localized at the applicator exit, that is near the walls;        on the other hand, the plasma produced by the fast electrons diffuses to either side of the trajectories of the fast electrons, that is perpendicularly to the magnetic field lines. As the probability of diffusing to one side or to the other of the trajectory of the fast electrons is the same, the probability for the plasma to diffuse toward the walls and that of diffusing away from the walls are statistically equal. It may then be considered that half the plasma produced by the fast electrons will be lost directly on the walls without filling the useful plasma volume situated beyond the trajectory moving away from the walls.        
One goal of the invention is therefore to design a microwave coaxial applicator making it possible to correct these two shortcomings.